The typical operating temperature for the reactors or combustors of such CFBs, and thus the flue gases produced thereby, lies within a temperature range of approximately 1550-1650°F. This temperature range thus lies within an acceptable temperature “window” for the application of selective non-catalytic reduction (SNCR) techniques for reducing NOx emissions, since SNCR systems and their associated apparatus typically involve the introduction of a specific reactant into flue gases whose temperature lies within a temperature range of approximately 1400-2000° F. In SNCR, a reducing agent or reactant, typically ammonia or urea, is sprayed into the furnace flue gas for reducing NOx according to one of the following reactions, depending upon the reactant employed:4NO+4NH3+O2→4N2+6H2O (ammonia-based)2NO+(NH2)2CO+½O22N2+2H2O+CO2 (urea-based).
SNCR is frequently used in CFB boilers which employ cyclone(s) for separating solids from the flue gas leaving the furnace to reduce NOx emissions. In such applications, the aforementioned reactant is sprayed at the inlet or outlet of the cyclone utilizing the high gas turbulence associated with the cyclone for mixing the flue gas with the reactant. These spray locations also take advantage of a relatively small cross-sectional flow area of the cyclone inlet or outlet, thereby allowing sufficient penetration of the jets of reactant into the flue gas flow to provide more uniform mixing of the reactant into the flue gas.
In contrast to the CFBs described above, another type of CFB reactor, combustor and/or boiler (hereinafter referred to as a CFB boiler for convenience) employs low velocity, impact-type particle separators, such as U-beams, for separating solids from the flue gas leaving the furnace and features a relatively large cross-sectional flow area for the flue gas flow. Utilizing nozzles to inject such reactants for SNCR which are installed only on the periphery of walls of the CFB which convey the flue gas flow might not achieve sufficient jet penetration of the reactant into the flue gas flow, resulting in poor mixing of the reactant with the flue gas.